This invention relates to a sheet material for heat transference, more particularly to a heat transfer sheet for carrying out heat printing in accordance with image information by means of thermal heads or the like and a heat transferable sheet (i.e., a sheet to be transferred) to be used in combination therewith, and also to a heat transfer recording process for forming an image by use of these sheets.
Heretofore, a heat-sensitive color-producing paper has been primarily used to obtain an image in accordance with image information by means of the contact type dot-shaped heating means such as thermal heads or the like. In this heat-sensitive color-producing paper, a leuco dye which is colorless or pale-colored at room temperature and a developer provided on a base paper are contacted by the application of heat to obtain a developed color image. Phenolic compounds, derivatives of zinc salicylate, rosins and the like are generally used as such a developer. However, the heat-sensitive color-producing paper as described above has a serious drawback in that its color disappears when the resulting developed color image is stored for a long period of time. Further, color printing is restricted to two colors, and thus it is impossible to obtain a color image having a continuous gradation.
On the other hand, a heat-sensitive transfer sheet wherein a heat-fusing wax layer having a pigment dispersed therein is provided on a base-paper has been recently used. When this heat-sensitive transfer sheet is laminated with a paper to be heat transfer printed, and then heat printing is carried out from the back of the heat-sensitive transfer sheet, the wax layer containing the pigment is transferred onto the heat transferable paper to produce an image. According to this printing process, an image having durability can be obtained, and a multi-color image can be obtained by using a heat-sensitive transfer paper each containing three primary color pigments and printing it many times. However, it is impossible to obtain an image having an essentially continuous gradation as in a photograph.
In recent years, there has been a growing demand for obtaining an image like a color photograph directly from an electrical signal, and a variety of attempts have been made to meet this demand. One of such attempts provides a process wherein an image is projected onto a cathode-ray tube (CRT), and a photograph is taken with a silver salt film. However, when the silver salt film is an instant film, the running cost is disadvantageously high. When the silver salt film is a 35 mm film, the image cannot be instantly obtained because it is necessary to carry out a development treatment after the photographing. An impact ribbon process and an ink jet process have been proposed as further processes. In the former, the quality of the image is inferior. In the latter, it is difficult to simply obtain an image like photograph because an image processing is required.
In order to overcome such drawbacks, there has been proposed a process wherein a heat transfer sheet provided with a layer of sublimable disperse dyes having heat transferability is used in combination with a heat transferable sheet, and wherein the sublimable disperse dye is transferred onto the heat transferable sheet while it is controlled to form an image having a gradation as in a photograph. (Bulletin of Image Electron Society of Japan, Vol. 12, No. 1 (1983)). According to this process, an image having continuous gradation can be obtained from a television signal by a simple treatment. Moreover, the apparatus used in the process is not complicated and therefore is attracting much attention. One example of prior art technology close to this process is a process for dry transfer calico printing polyester fibers. In this dry transfer calico printing process, dyes such as sublimable dispersed dyes are dispersed or dissolved in a solution of synthetic resin to form a coating composition, which is applied onto tissue paper or the like in the form of a pattern and dried to form a heat transfer sheet, which is laminated with polyester fibers constituting sheets to be heat transferred thereby to form a laminated structure, which is then heated to cause the disperse dye to be transferred onto the polyester fibers, whereby an image is obtained. However, even if the heat transfer sheet heretofore used in the dry transfer calico printing process for the polyester fibers is used as it is and subjected to heat printing by means of thermal heads or the like, it is difficult to obtain a developed color image of a high density.
While improvement of the image quality due to printing density and heat sensitivity is an important task in the prior art technology as described above, another important point which is the problem in the practical process of forming a heat transferred image is the operability in the printing step. To describe about this operability, the following problems have been involved in the sheet for heat transference of the prior art.
(a) In the heat transfer sheet of the prior art, when the sheet is conveyed by means of a printing conveying means, the sheet may be sometimes adhered to the roll within the means, whereby running performance of the heat transfer sheet becomes worse.
(b) In the heat transfer sheet of the prior art, the so-called sticking phenomenon occurs, in which the base sheet itself is .fused to the thermal heads, whereby running of the heat transfer sheet may become impossible or, in an extreme case, the sheet may be broken from the sticked portion.
(c) In the sheet of the prior art, dust may be inhaled through the electrostatic attracting force created by running or friction of the sheet, whereby disadvantages such as dislocation of recording (partial failure of recording), damages of the dot-shaped heat printing means such as thermal heads or the like, bad running performance such as sagging of respective sheets, etc., caused by attachment of dust between the heat transfer sheet and the heat transferable sheet or between the dot-shaped heat printing means and the heat transfer sheet remain as problems to be solved.
(d) In the heat transferable sheet of the prior art, running performance of the sheet is bad depending on the base sheet employed and, further, the strain created by the heat during image formation disadvantageously remains on the sheet to cause curling of the sheet.
(e) For formation of a color image by heat-sensitive transfer printing, a heat-sensitive transfer sheet in which transfer layers are provided by coating in different areas of a plurality of colors has been invented. However, even such layers may be provided by coating in different areas, there is no guarantee that the area of a desired color can be heat printed and therefore it is necessary to confirm the transfer layer every time of heat printing. Also, in the case of a monochromatic heat-sensitive transfer sheet, it has been inconveniently impossible to confirm the residual amount, the direction, back or front, grade, etc. of the heat-sensitive transfer sheet.
(f) The heat transferable sheet of the prior art is ordinarily a merely white sheet in appearance and therefore, even a paint prepared from various resins, optionally with addition of additives, may be applied in one layer or multiple layers, it is difficult to discriminate one from another with naked eyes. Not only distinction from papers for other recording systems such as electrostatic copying paper or heat-sensitive recording paper or the like, as a matter of course, but also distinction between several kinds of heat transferable sheets depending on adaptability for recording devices or heat transfer sheets or uses are greatly required.
However, in the prior art, once this kind of heat transferable sheet is unwrapped from a package, distinction from appearance is hardly possible and yet no measure for distinction has been taken.